The present invention relates to an etching method and an etching apparatus employed when practicing this etching method.
To accurately form fine electrodes and wirings in various devices such as semiconductor devices and magnetic bubble memory devices, so-called dry etching is often employed in which an article to be etched and a reaction gas are brought into contact with each other to etch the article.
Among various dry etching methods, one type of dry etching method is most widely adopted which employs parallel plate type etching equipment in which two parallel metallic electrodes are disposed in opposing relation to each other, and an article to be etched is placed on one of these electrodes to effect etching.
This type of dry etching method is known as "reactive sputter etching" or "reactive ion etching" and has the advantage that it is possible to suppress the occurrence of defects known as "side etch" (or "undercut"), that is, a portion of the article which is under the mask is undesirably etched, and that etching can be effected with a high degree of accuracy.
However, as the packing density of devices such as semiconductor devices and magnetic bubble memory devices further increases and the dimensions of electrodes and wirings further decreases, there is a demand for a dry etching technique which enables these extremely fine electrodes and wirings to be formed with a high degree of accuracy.
For this reason, microwave plasma etching has already been proposed wherein a plasma is generated using a microwave to effect etching (see, Japanese Patent Publication No. 37311/1981).
As compared with the above-described reactive ion etching which employs parallel plate type etching equipment, the microwave plasma etching enables electric discharge to occur even at a relatively low pressure (1.times.10.sup.-3 Torr or less) and therefore has a large number of advantages. For example, it is possible to generate a high-density plasma in which ions are arranged in one direction, and to effect highly accurate etching with less defects, such as side etch. However, the microwave plasma etching still needs a number of improvements to be made in order to practically apply it widely in the manufacture of semiconductor devices and the like.
More specifically, when a polycide film (a two-layer film consisting of a silicide film of a metal such as tungsten and molybdenum and a polycrystalline silicon film), which is employed to form a gate electrode or wiring of a semiconductor device having a high packing density, is etched into a desired configuration, it is required that the dimensional shift from the mask dimensions should not be large, and the etch rate of the oxide film formed under the polycide film should not be large. However, no etching technique which can satisfy the above-described requirements has been found in the conventional reactive ion etching method.
Further, in the conventional microwave plasma etching method, when the etching selectivity ratio is increased, it is not possible to effect vertical etching (etching in which side etch is reduced, and the side wall is substantially vertical), whereas, when it is intended to perform vertical etching, the selectivity ratio must be decreased. More specifically, in any of the above-described reactive sputter etching, reactive ion etching and microwave plasma etching, although the selectivity ratio is relatively high in a gas plasma of a compound containing fluorine, vertical etching is almost impossible, and in a gas plasma of a compound containing chlorine or a gas plasma of a mixed gas formed by adding H.sub.2 or NH.sub.3 into a gas of a compound containing fluorine, the selectivity ratio cannot be increased although vertical etching can be effected. For example, in the conventional bias application type microwave plasma etching, since a bias voltage is applied to an article to be etched at all times, the ion energy is relatively high, and the etch rate of the oxide film which constitutes the foundation is therefore relatively large, so that it is difficult to leave the underlying oxide film unetched which is exposed by the etching of the polycide film. In addition, it is often that a thin film (side wall film) cannot be formed on the side wall formed by etching although it depends on the kind of employed gas. Since it is necessary in order to effect vertical etching to prevent side etching by means of the above-described side wall film, if the side wall film is not formed on the side wall, it becomes impossible to effect vertical etching.